1. Field of the Invention
The present invention generally relates to projection and display systems, and more particularly to an overhead projector having a computer integrated therein with several output ports for external devices, and further providing a unique interface card which electronically connects the overhead projector to a liquid crystal display panel and simultaneously aligns the panel with the stage of the overhead projector.
2. Description of the Prior Art
Overhead projectors (OHP's) are known in the art, and generally consist of a base having a stage area, a light source to illuminate the stage, and a projector head which houses mirrors and lenses designed to project any image placed on the stage area onto a display screen. OHP's may be either transmissive or reflective. In a transmissive OHP, the light source is under the stage area, i.e., opposite the side of the projector head. In a reflective OHP, the light source is above the stage area, typically near or even in the projector head, and the stage has a mirror-like surface to reflect the light back toward the head.
Use of OHP's has recently expanded from the traditional projection of images printed on transparent films, to the projection of images which are stored in machine-readable form (e.g., on magnetic or optical disks) and which are presented by means of a light-transmissive liquid crystal display (LCD) panel which is laid on the stage of the OHP. Exemplary LCD/OHP arrangements are shown in U.S. Pat. Nos. 4,846,694 and 4,944,578. The LCD panels are controlled by external processors, such as a personal computer (PC). This type of system enhances the use of an OHP since the LCD controller may simultaneously direct the video output to other viewing devices, such as television monitors, which may be placed at several stations among the audience.
Several problems have arisen, however, in the use of LCD panels with OHP's. Foremost among these is the difficulty of storing and transporting the PC which controls the LCD panel, and connecting the PC to the panel. Three alternative systems have been devised to overcome this problem. The first system simply provides a portable "black box," much more compact than a normal PC and having simplified user inputs, which contains the minimum electronic components necessary to control the LCD panel, such as a microprocessor, memory storage and a monitor output port. An example of such a system is VIDEOSHOW (a trademark of General Parametrics Corp. of Berkeley, Calif.), wherein the graphic images are prepared at a normal PC, and then transferred to a magnetic (floppy) diskette which may be placed in the portable unit. The second system similarly incorporates certain components into the LCD panel, instead of the OHP. An example of an LCD panel having a microprocessor and memory means for storing images is disclosed in European patent application no. 89114916.3.
A third system for simplifying the LCD/OHP combination integrates certain computer components within the OHP. An example of this type of system is found in U.S. Pat. No. 4,882,599. This design, however, suffers from the drawback of the processor being limited to the single purpose of controlling the LCD panel. In other words, it is impossible to adapt the integrated OHP-computer for other uses such as remote communications, printing, etc. More generally, these functions are unavailable due to the lack of any external ports for connection of peripherals, such as a modem, printer, mouse, etc. Another disadvantage is that these systems do not support direct digital drive (for VGA-compatible panels), so that video signals must be converted from digital to analog and then back to digital again to interface to the LCD. Also, prior art OHP-computers are not compatible with conventional PC operating systems, such as PC-DOS, OS/2 (trademarks of International Business Machines), MS-DOS (a trademark of Microsoft Corp.), DR DOS (a trademark of Digital Research, Inc.), or MACINTOSH system (a trademark of Apple Computers, Inc.), additionally limiting their usefulness.
A further inconvenience of using an LCD panel with an OHP relates to the physical attachment of the panel to the OHP, and the electrical connections between the panel, its controller, and the OHP. Those skilled in the art will appreciate the importance of properly aligning the LCD panel with the stage of the overhead projector. Misalignment can result in lower light transmission through the panel, leading to an inferior projected image, and heat management problems; this is particularly critical in stacked panel designs which are subject to parallax problems, and also when special optical components, such as fresnel lenses, are used with the OHP. While it is not terribly difficult to properly align a portable LCD panel on an OHP, the fumbling that often occurs in this step adversely detracts from the presentation, and this problem can be amplified depending upon the particular dimensions of the LCD panel and OHP being used. With respect to the problem of electrical interconnection, anyone who has made a presentation with an LCD/OHP system has experienced the confusion of trying to connect the various cables needed for power, video, LCD control and other accessories. This confusion leads not only to delays, but also presents the serious hazard of electrical shock to a user or damage to the equipment if the cabling is not properly connected. Additionally, the user must insure that the proper cables (i.e., terminal connectors) are provided for the particular computer/LCD ports being used.
The problems mentioned in the foregoing paragraph are avoided in systems wherein the LCD panel is permanently integrated with the OHP. See, e.g., U.S. Pat. Nos. 4,763,993 (FIG. 4b) and 4,880,303. The first of these devices, however, is less desirable since the LCD panel always remains in the optical projection path, even when a transparency, rather than the LCD panel, is being used to provide the image. This in turn results in lower light transmission (due to the use of polarizers in the LCD panel) and hence poorer contrast in the projected image. In the second of these devices, although the LCD panel can be hinged upwardly out of the optical projection path, its attachment to the OHP still leads to storage and transportation problems and, in both devices, the LCD panel is not removable and so cannot be used with other systems. The latter aspect of these devices can be considerably frustrating, e.g., when the LCD panel is properly functioning but the OHP is nonfunctional, which may have many causes including breakdown of electrical components/connections in the OHP, or breakage of optical components such as the lamp or lenses. It would, therefore, be desirable and advantageous to devise an LCD/OHP system which would overcome all of the above limitations.